Strong, uniform magnetic fields are desirable in many situations. Examples include scientific instruments, devices for measuring important industrial parameters, apparatus for achieving certain effects on samples or materials, devices for education or student experimentation in courses, and many others. Often, the effectiveness of a device or phenomenon that relies on, or incorporates, a magnetic field in its operation will be improved if the uniformity of the field strength is enhanced. Such operational improvements may be increases in speed, efficiency, precision, resolution, sensitivity, or enhancements to other features or functions of the device, measurement, or process. A more uniform magnetic field may also allow the use of smaller, weaker, and/or less expensive magnets to achieve the desired goal.
Many methods for homogenizing or correcting the magnetic field of magnets have been developed. Mechanical methods, including precise design and shaping of the magnet's pieces, attention to accurate positioning during the assembly of the magnet, careful selection of uniform and/or matched pieces of permanent magnetic material, and careful adjustment of the relative positions of the magnet pieces after initial assembly, have been used. Post-assembly mechanical adjustments, including screws to move some or all of the positions of the magnet's constituent pieces, movement of magnet pieces that control the distribution of magnetic flux lines in space, additions of extra pieces of material on the surface of or in the interior and/or exterior of the magnet to change the magnetic field, additions of smaller pieces of magnetic material to cause localized increases or decreases in field strength, and large-scale distortions of the magnet structure are all examples of mechanical methods for improving the field homogeneity of magnets. This list of mechanical methods is not exhaustive. For example, magnets may be disassembled and then reassembled with new thin pieces or “shims” incorporated so that the relative positions of the magnet's constituents are altered, changing the shape of the magnetic field. Older examples of past efforts to adjust, with mechanical shimming, a magnetic field include U.S. Pat. No. 4,631,481 to Young et al., U.S. Pat. No. 5,235,284 to Tahara et al., U.S. Pat. No. 5,343,183 to Shimada et al., and U.S. Pat. No. 6,313,634 to Kasten.
Non-mechanical methods are likewise possible. The non-mechanical methods often have the attractive characteristic of continuous adjustability, ease of adjustment, and ease of automation. One very common non-mechanical method utilizes wires that carry electrical current. Such currents create magnetic fields around the wires. These fields are strongest near the wires and have a spatial distribution of strength and orientation that can be calculated from known physical principles. The wires sometimes are configured as coils, and are placed more or less proximate to the magnetic field to be corrected. By carefully adjusting the currents in such wires, the homogeneity of the “target” or original magnetic field can be increased, because the magnetic fields generated by the wire currents can counteract some, or most, of the inhomogeneities present in the field of the original magnet. By analogy with the thin sheets or strips of metal (e.g., shim stock) that have sometimes been used to mechanically adjust magnets, these coils of wire have come to be known as “shims” or “shim coils.” Because the individual coils of wire used to correct the original magnetic field typically create non-uniform fields, they are also sometimes referred to as “gradients” or “gradient coils.” Other names may be employed. An example of a relatively recent disclosure in the field of electronic wire coil shims is U.S. Patent Application Publication No. 2011/0137589 by Leskowitz et al., the entire disclosure of which is incorporated herein by reference by way of background.
Non-mechanical methods may be used in conjunction with one or more of the mechanical methods. Mechanical adjustments are often used to correct the inhomogeneities that are large in amplitude or extend over a large length-scale, with non-mechanical adjustments used for fine correction of the residual field defects.